1. Field of the Invention
The present invention relates to an improved method for separating drops of molten liquid, e.g., urea, contained in a gas stream discharged from a process wherein a melamine synthesis effluent gas containing melamine obtained from the thermal decomposition of urea or of thermal decomposition products of urea is cooled to separate melamine by crystallization, and the resulting effluent gas having melamine separated is closely contacted with a molten mass of, e.g., urea or a mixture of urea and thermal decomposition products thereof for recovering the unreacted urea and the like.
2. Description of the Prior Art
It is well known in the art that melamine is produced by heating urea or thermal decomposition products thereof with ammonia in the presence of a catalyst. There is also known a method for separating melamine, unreacted urea and thermal decomposition products of urea from the synthesis effluent gas containing melamine as disclosed in Japanese Patent Publication No. 21343/1966. This method comprises the steps of mixing the melamine synthesis effluent gas obtained by catalytically heating urea and the like with an inert gas thereby cooling the mixture to a temperature at which condensation of unreacted urea does not occur to selectively crystallize melamine so that only melamine is separated from the gas, closely contacting the effluent gas after the separation and still containing carbon dioxide, ammonia, melamine vapor and water vapor with molten urea or a molten mixture of urea and thermal decompositon products thereof maintained at a temperature above the melting point of urea to recover the unreacted urea and melamine vapor contained in the gas by dissolving or mixing them into said molten mass, and recirculating at least a portion of the gas stream from the recovery step as said inert gas for crystallization and separation of melamine.
In the above-described method, however, liquid drops composed of molten urea or of a molten mixture of urea and thermal decomposition products thereof that has been used for washing tend to be contained in the waste gas from the contacting step. Since this waste gas is mixed with the melamine synthesis effluent gas to cool the same, the aforementioned liquid drops are incorporated into the resultant crystallized melamine causing deterioration of the quality thereof. It is, therefore, necessary to remove said liquid drops prior to mixing of the waste gas with the melamine synthesis gas.
The separated liquid drops, however, adhere to the inner wall of the separator used and solidify thereon when the gas containing liquid drops is passed to a liquid-vapor separator for removing the liquid drops therefrom. This method has a disadvantage in the thus adhered mass which increases in amount with the passage of time, resulting in considerable lowering of the efficiency of the liquid-vapor separation and difficulties in continuous operation for an extended period of time. To overcome this disadvantage, it has been proposed in French Patent No. 1,560,175 to use a separator packed with Raschig rings and to flood the separator from time to time with molten urea to prevent formation of the solid incrustation. This method, however, has the disadvantage that it requires a plurality of liquid-vapor separators to be employed one at a time so that the installation cannot be operated continuously, thus rendering the operation considerably more complicated.
It is therefore an object of the present invention to provide a method of separating liquid urea drops from a waste gas from a washing step following melamine synthesis which ensures a continuous operation for an extended period of time and also facilitates the operation.
Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific example, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It has been found that the above objects can be attained by practicing the present invention.